On the Generation of l/f Noise in Black Hole Objects

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There exist short-term fluctuations with a frequency dependence of f^<-beta> with beta-1 in the X-rays from stellar black-hole candidates and active galactic nuclei.The amplitude (or the peak intensity) of the fluctuations shows a rather smooth distribution without any typical values, just like solar flares.The basic features of the fluctuations can be understood, if the inner portions of the disk is in a self-organized state ; when the mass density of the disk exceeds some critical value, an unknown instability is assumed to become established and accumulated material begins to drift inward as an avalanche, thereby emitting X-rays.As to the origin of the assumed instability, the reconnection of disk magnetic fields, resulting in flares similar to solar flares, is the most promising candidate. On the basis of this working hypothesis, we conjecture that a black-hole accretion disk oscillates between a solar-type (high-beta) disk with small variabilities and a coronal (low-beta) disk with large fluctuations.Magnetic fields emerge via buoyancy in the from of loo-like structures in the former, whereas the latter is composed of blobs threaded by mainly toroidal fields.